Transfer Laminate, Method for Manufacturing Transfer Laminate, Method for Manufacturing Electrode for Lithium Secondary Battery, and Lithium Secondary Battery

This application claims priority to and the benefit of Korean Patent Application No. 10-2022-0175738 filed in the Korean Intellectual Property Office on Dec. 15, 2022, the entire contents of which are incorporated herein by reference.

The present application relates to a transfer laminate, a method for manufacturing a transfer laminate, a method for manufacturing an electrode for a lithium secondary battery, and a lithium secondary battery.

Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy or clean energy is increasing, and as part thereof, the fields that are being studied most actively are the fields of power generation and power storage using an electrochemical reaction.

At present, a secondary battery is a representative example of an electrochemical device that utilizes such electrochemical energy, and the range of use thereof tends to be gradually expanding.

Along with the technology development and the increase in demand for mobile devices, the demand for secondary batteries as an energy source is sharply increasing. Among such secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle life, and low self-discharging rate have been commercialized and widely used. In addition, research is being actively conducted on a method for manufacturing a high-density electrode having a higher energy density per unit volume as an electrode for such a high-capacity lithium secondary battery.

In general, a secondary battery includes a positive electrode, a negative electrode, an electrolyte solution, and a separator. The negative electrode includes a negative electrode active material for intercalating and deintercalating lithium ions coming out from the positive electrode, and silicon-based particles having a high discharge capacity may be used as the negative electrode active material.

In general, a carbon material such as graphite is used for a negative electrode of a lithium secondary battery, but the theoretical capacity density of carbon is 372 mAh/g (833 mAh/cm). Therefore, in order to improve the energy density of the negative electrode, silicon (Si), tin (Sn), and oxides and alloys thereof, which are alloyed with lithium, are considered as negative electrode materials. Among them, silicon-based materials have attracted attention due to their low cost and high capacity (4200 mAh/g).

However, when a silicon-based negative electrode active material is used, a problem arises in that the initial irreversible capacity is large. In the charging and discharging reactions of the lithium secondary battery, lithium discharged from the positive electrode is intercalated into the negative electrode during charging, and is deintercalated from the negative electrode to return to the positive electrode again during discharging. In the case of the silicon-based negative electrode active material, the volume change and the surface side reaction are severe, so that a large amount of lithium intercalated in the negative electrode during initial charging does not return to the positive electrode again, and thus an initial irreversible capacity increases. When the initial irreversible capacity increases, there occurs a problem that the battery capacity and the cycle are rapidly reduced.

In order to solve the above problems, known is a method for pre-lithiating a silicon negative electrode including a silicon-based negative electrode active material. As the pre-lithiation method, known methods include a method of manufacturing an electrode after lithiation by a physicochemical method such as electrolytic plating, lithium metal transfer, and lithium metal deposition, a method of electrochemically pre-lithiating a negative electrode, and the like.

In order to use the existing electrochemical method, the wet process should be performed within the electrolyte, which poses risks such as fire and explosion, so it was necessary to control an inert environment well. That is, when creating the environment, it is difficult to control conditions such as moisture control by using inert gas in a room where the electrochemical method is performed. In addition, in order to uniformly control the initial irreversible capacity, a rate of pre-lithiation should be slowed down as much as possible using an electrochemical method, so a problem arises in that the production cost increases in the application of the electrochemical method.

In a lithium metal transfer process that is a different method, it is difficult to transfer lithium metal safely and easily, and lithium is not transferred from a transfer laminate, or even if lithium metal is transferred, the highly reactive lithium metal immediately starts reacting with a negative electrode active material, thereby causing problems such as particle breakage on a surface of the negative electrode active material layer.

In particular, in the transfer-type pre-lithiation process, it is important to secure transferability of a lithium metal layer, and application for mass production is possible only when the transferability is secured. Research is underway to easily transfer the lithium metal layer from the transfer laminate, but a method of transferring the lithium metal layer more safely and faster has not been found yet.

Therefore, there is a need for research on a transfer laminate that can enable lithium to be uniformly pre-lithiated in an electrode active material layer more safely and efficiently when pre-lithiating an electrode.

In the transfer-type pre-lithiation process, a technology that can easily transfer a lithium metal layer on top of an electrode active material layer is essential. Accordingly, methods such as including a release layer or controlling adhesive force were studied, but could not solve clearly the above-described problems. In the present application, it was found through research that when a temperature of a base material layer is adjusted in a process of forming a lithium metal layer on the base material layer, a ratio of a lithium element and an oxygen element in a surface and a specific region of the lithium metal layer can be adjusted and that when the ratio is adjusted, the lithium metal layer can be easily transferred.

Accordingly, the present application relates to a transfer laminate, a method for manufacturing a transfer laminate, a method for manufacturing an electrode for a lithium secondary battery, and a lithium secondary battery.

An exemplary embodiment of the present specification provides a transfer laminate including a base material layer; and a lithium metal layer stacked on one surface or both surfaces of the base material layer, wherein a thickness of the lithium metal layer is 1 μm or greater and 20 μm or less, and wherein a first region including a thickness of 1 nm or greater and 500 nm and less based on a surface of the lithium metal layer opposite to a surface facing the base material layer satisfies Formula 1 below.

in Formula 1 above,

Another exemplary embodiment provides a method for manufacturing a transfer laminate including: preparing a base material layer; and forming a lithium metal layer on one surface of the base material layer by heating and depositing a lithium source, wherein a surface temperature of the base material layer in the step of forming of the lithium metal layer is 90° C. or lower.

Still another exemplary embodiment provides a method for manufacturing an electrode for a lithium secondary battery, the method including: forming an electrode current collector layer and an electrode active material layer on one surface or both surfaces of the electrode current collector layer; and transferring a lithium metal layer onto the electrode active material layer, wherein the transferring of lithium metal layer includes preparing the transfer laminate according to the present application, laminating the transfer laminate onto the electrode active material layer such that a surface of the lithium metal layer opposite to a surface facing the base material layer comes into contact with a surface of the electrode active material layer opposite to a surface in contact with the electrode current collector layer, and removing the base material layer.

Finally, provided is a lithium secondary battery including: a positive electrode for a lithium secondary battery; a negative electrode for a lithium secondary battery; a separator provided between the positive electrode and the negative electrode; and an electrolyte, wherein at least one of the positive electrode for a lithium secondary battery and the negative electrode for a lithium secondary battery is the electrode for a lithium secondary battery manufactured according to the method described above.

The transfer laminate according to an exemplary embodiment of the present invention is a transfer laminate that is used in a transfer-type pre-lithiation process. In particular, the transfer laminate includes the lithium metal layer formed on the top of the base material layer by heating and depositing a lithium source. In this case, the surface temperature of the base material layer in the step of forming the lithium metal layer is adjusted to 90° C. or lower.

According to the characteristics of the manufacturing method as described above, the transfer laminate according to the present application satisfies the range of Formula 1 above in the first region including the thickness of 1 nm or greater and 500 nm and less based on the surface of the lithium metal layer opposite to the surface facing the base material layer.

In the transfer laminate, the composition near the surface of the lithium metal layer satisfies the range of Formula 1 above, so it is possible to ensure transferability when transferring the lithium metal layer to an electrode later by adjusting the ratio of oxygen in the surface of the lithium metal layer. That is, the surface of the lithium metal layer ultimately comes into contact with a surface of a transfer target to which the lithium metal layer is transferred, and in this case, by adjusting the ratio of oxygen in the surface, the reactivity issue with the transfer target and the transfer force can be adjusted during transfer, so productivity is secured and transferability can be improved in R2R-type pre-lithiation.

That is, the transfer laminate according to the present application has the adjusted composition of the lithium metal layer. Accordingly, the lithium transferability during the pre-lithiation process can be improved, resulting in suppression of generation of by-products during the pre-lithiation.

Before describing the present invention, some terms are first defined.

When one part “includes”, “comprises” or “has” one constituent element in the present specification, unless otherwise specifically described, this does not mean that another constituent element is excluded, but means that another constituent element may be further included.

In the present specification, ‘p to q’ means a range of ‘p or more and q or less’.

In this specification, the “specific surface area” is measured by the BET method, and specifically, is calculated from a nitrogen gas adsorption amount at a liquid nitrogen temperature (77K) by using BELSORP-mini II available from BEL Japan, Inc. That is, in the present application, the BET specific surface area may refer to the specific surface area measured by the above measurement method.

In the present specification, “Dn” means an average particle diameter, and means a particle diameter at the n % point in the cumulative distribution of the number of particles according to the particle diameter. That is, D50 is a particle diameter at the 50% point in the cumulative distribution of the number of particles according to the particle diameter, D90 is a particle diameter at the 90% point in the cumulative distribution of the number of particles according to the particle diameter, and D10 is a particle diameter at the 10% point in the cumulative distribution of the number of particles according to the particle diameter. Meanwhile, the average particle diameter may be measured using a laser diffraction method. Specifically, after powder to be measured is dispersed in a dispersion medium, the resultant dispersion is introduced into a commercially available laser diffraction particle size measurement apparatus (for example, Microtrac S3500) in which a difference in a diffraction pattern according to the particle size is measured, when a laser beam passes through particles, and then a particle size distribution is calculated.

In the present specification, the description “a polymer includes a certain monomer as a monomer unit” means that the monomer participates in a polymerization reaction and is included as a repeating unit in the polymer. In the present specification, when a polymer includes a monomer, this is interpreted as the same as that the polymer includes a monomer as a monomer unit.

In the present specification, it is understood that the term ‘polymer’ is used in a broad sense including a copolymer unless otherwise specified as ‘a homopolymer’.

In the present specification, a weight-average molecular weight (Mw) and a number-average molecular weight (Mn) are polystyrene converted molecular weights measured by gel permeation chromatography (GPC) while employing, as a standard material, a monodispersed polystyrene polymer (standard sample) having various degrees of polymerization commercially available for measuring a molecular weight. In the present specification, a molecular weight refers to a weight-average molecular weight unless particularly described otherwise.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings so that one skilled in the art can readily implement the present invention. However, the present invention may be embodied in various different forms, and is not limited to the following descriptions.

An exemplary embodiment of the present specification provides a transfer laminate including a base material layer; and a lithium metal layer stacked on one surface or both surfaces of the base material layer, wherein a thickness of the lithium metal layer is 1 μm or greater and 20 μm or less, and wherein a first region including a thickness of 1 nm or greater and 500 nm and less based on a surface of the lithium metal layer opposite to a surface facing the base material layer satisfies Formula 1 below.

in Formula 1 above,

In the transfer laminate according to the present invention, the composition near the surface of the lithium metal layer satisfies the range of Formula 1 above, so it is possible to ensure transferability when transferring the lithium metal layer to an electrode later by adjusting the ratio of oxygen in the surface of the lithium metal layer. That is, the surface of the lithium metal layer ultimately comes into contact with a surface of a transfer target to which the lithium metal layer is transferred, and in this case, by adjusting the ratio of oxygen in the surface, the reactivity issue with the transfer target and the transfer force can be adjusted during transfer, so productivity is secured and transferability can be improved in R2R-type pre-lithiation.

In an exemplary embodiment of the present application, the base material layer can be used without limitation as long as it has features capable of withstanding process conditions such as high temperature in the step of depositing the lithium metal layer and preventing a reverse peeling problem that the lithium metal layer is transferred onto the base material layer during a winding process for transferring the deposited lithium metal.

Specifically, in an exemplary embodiment of the present application, the base material layer may be one or more selected from the group consisting of polyethylene terephthalate (PET), polyimide (PI), poly(methylmethacrylate) (PMMA), polypropylene, polyethylene and polycarbonate.

In an exemplary embodiment of the present application, a thickness of the base material layer may be 1 μm or greater and 300 μm or less, and may satisfy a range of 5 μm or greater and 200 μm or less, and 10 μm or greater and 100 μm or less.

In an exemplary embodiment of the present application, a thickness of the lithium metal layer may be 1 μm or greater and 50 μm or less, and may satisfy a range of 3 μm or greater and 25 μm or less.

As the thickness of the base material layer satisfies the above range, transfer of the lithium metal layer to the electrode active material layer can occur efficiently, and in particular, when the base material layer has the above range, heat dissipation can occur effectively, resulting in preventing problems of reverse transfer and generation of by-products during pre-lithiation.

In an exemplary embodiment of the present application, a release layer may be further included on a surface in contact with the base material layer and the lithium metal layer of the transfer laminate, in order to improve the peelability of the lithium metal layer, to secure transferability to the electrode active material layer and to serve as a protective layer after transfer of the lithium metal layer.

That is, the base material layer may have a release layer formed on at least one surface, or may have release layers formed on both surfaces. The release layer makes it possible to prevent a reverse peeling problem that the lithium metal layer is transferred onto the base material layer during a winding process for transferring the deposited lithium metal layer to an electrode, and also makes it possible to easily separate the base material layer after transferring lithium metal onto the electrode active material layer.

In an exemplary embodiment of the present application, a thickness of the release layer may be 1 nm or greater and 1 μm or less.

In another exemplary embodiment, the thickness of the release layer may satisfy a range of 1 nm or greater and 1 μm or less, preferably 100 nm or greater and 1 μm or less, and more preferably 500 nm or greater and 1 μm or less.

The release layer satisfies the above thickness range and satisfies a specific adhesive force range. The release layer has the above thickness range, so that a range of adhesive force with an upper part of the pre-lithiated electrode after pre-lithiation can be adjusted to a lower limit of a certain range, and therefore, side reactions between the release layer and the electrode active material layer are prevented.

The release layer may include one or more species selected from the group consisting of silicon-modified polyester in which a silicon chain is graft-linked to a polyester main chain, an acrylic resin, Si, melamine, and fluorine.